If your end goal is gvim then you will have your work cut out for you, since there are a number of unkeyworded packages that need to be compiled to get to gvim. If you just need vim, I think that OS X 10.3 comes with vim 6.2, which you might be able to use with gvim. In order to see everything that leads to gvim, you can emerge -pv gvim.

Also since your edits will disappear with the next sync, it is best not to directly edit the ebuild; if you are interested in compiling a specific version of a package then you should add it to /etc/portage/package.unmask:
echo "=app-editors/vim-core-6.3-r2" >> /etc/portage/package.unmask.